Embodiments of the invention relate generally to radiation detectors and, more particularly, to an apparatus and method for acquiring and processing electronic data from a radiation detector.
In the fields of security screening and medical imaging, non-invasive imaging techniques employing radiation detectors have gained importance due to benefits that include unobtrusiveness, ease, and speed. A number of non-invasive imaging techniques exist today. Single-photon-emission computed tomography (SPECT) imaging and x-ray computed tomography (CT) imaging are two examples.
At least two factors explain the increased importance of radiation detectors in security screening: an increase in terrorist activity in recent years, and an increase in the number of travelers. The detection of contraband, such as explosives and radioactive materials, being transported in luggage, cargo containers, and small vehicles and taken onto various means of transportation has become increasingly important. To meet the increased need for such detection, advanced systems have been developed that can not only detect suspicious articles being carried in luggage and other containers but can also determine whether or not the articles contain explosives or radioactive materials.
There is also a need for high-resolution gamma radiation detectors which can detect radioactive materials from a variety of sources. To gain widespread use, these radiation detectors must be economical, easily portable, and have low-power consumption. Semiconductor materials, such as cadmium-telluride (CdTe) and cadmium-zinc-telluride (CZT) crystals have applicability for compact radiation detectors. CdTe and CZT detectors have been shown to exhibit good energy resolution, especially as compared to scintillator-based detectors. Since they are direct conversion devices (i.e., convert radioactive particles, such as photons, directly into electronic signals), CdTe and CZT detectors eliminate the need for bulky photomultiplier tubes or the need for a scintillator as in a CT application. Furthermore, CdTe and CZT radiation detectors do not require cryogenic cooling, as do high-purity germanium radiation detectors.
SPECT and CT imaging systems can incorporate such semiconductor, or solid state, radiation detector technology. CT systems are capable of acquiring mass and density information (as well as materials-specific information, such as an effective atomic number) on items within a piece of luggage. Although object density is an important quantity, surrogates such as “CT number” or “CT value”, which represent a linear transformation of the density data, may be used as the quantity indicative of a threat. Features such as mass, density, and effective atomic number embody derived quantities such as statistical moments, texture, etc. of such quantities.
In CT imaging systems, an x-ray source emits a fan-shaped beam towards a subject or an object, such as, for example, a patient or piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the object. Each detector element of the detector array produces a separate electrical signal indicative of the strength of the attenuated beam received by each detector element. The electrical signals are transmitted from the detector array to a data processing system for analysis which ultimately produces an image.
Typically, in SPECT imaging systems, a gamma camera or similar radiation detector locates radiation emitted from a subject such as a patient, or an object such as a piece of luggage containing a radioactive substance. As above, “subject” and “object” are used interchangeably. When imaging a patient, a gamma-ray-emitting tracer material is administered to the patient. Typically, the tracer material is absorbed by the organ of interest to a greater degree than by other organs. In these systems, each element of the detector array produces a signal in relation to the localized intensity of the radiation emitted from the object. As with conventional x-ray imaging, the strength of the emission signal is attenuated by the inter-lying object or body part. Each element of the detector array produces a separate electrical signal indicative of the photon impinging upon the detector element. The electrical signals are transmitted from the detector assembly to a data processing system for analysis, which ultimately produces an image.
In SPECT imaging, a plurality of images is acquired at various angles around the area of interest. To acquire the images, the gamma camera is rotated around the object. Generally, in transaxial tomography, a series of 2-D images, or views, are taken at equal angular increments around the object. Typically, projections are acquired every 3-6 degrees. In some cases, a full 360 degree rotation is used to obtain an optimal reconstruction. Multi-head gamma cameras can provide accelerated image acquisition. For example, a dual-head camera can be used with detectors spaced 180 degrees apart, allowing two projections to be acquired simultaneously, with each head requiring 180 degrees of rotation. Triple-head cameras with 120 degree spacing are also used.
The series of views around the object are reconstructed to form transaxial slices, or slices across the axis of rotation. The reconstruction is performed by a computer, which applies a tomographic reconstruction algorithm to the multiple views, yielding a 3-D dataset. This dataset may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as CT and PET.
A gamma camera radiation detector assembly may employ a multi-channel collimator and gamma ray detector to convert energy from the gamma ray photon into an electrical signal, which can be interpreted to locate the position of the gamma ray interaction in a planar detector. Gamma cameras may also include a large scintillation crystal responsive to radiation stimuli, such as gamma rays, emitted by a patient, and an array of photomultiplier tubes optically coupled to the crystal. In operation, the gamma rays emitted by the patient in the direction of the detector are collimated onto the crystal. Each gamma ray photon cloud that interacts with the crystal produces multiple light events that are detected by the photomultipliers near the point of interaction. Each light event detected by the photomultipliers produces an electrical signal. The electrical signals from the photomultiplier array are combined to provide an estimate of the location of the gamma ray emission. Analog and digital processing of the signal results in the generation of an image from the acquired data.
However, gamma cameras may also employ semiconductor detector elements, such as cadmium-zinc-telluride (CZT) elements, to replace the scintillator/photomultiplier system. CZT detector elements convert the signal from gamma ray photons directly into an electronic signal. By eliminating the light conversion step needed in scintillator/photomultiplier cameras, a gamma camera using semiconductor radiation detectors may exhibit higher signal to noise ratio, and increased sensitivity which can result in greater energy level resolution and better imaging contrast resolution.
SPECT and CT imaging systems incorporating semiconductor detector array technology may be able to provide compositional analysis of tissue using spectroscopic x-ray imaging while improving overall image quality and reducing the x-ray dose to the object. Recent advances in the development cadmium-zinc-telluride (CZT) detectors and other direct conversion (i.e. semiconductor) detectors have extended the application of such detectors to medical imaging (i.e., SPECT and CT systems), security screening, nuclear experimentation, as well as to oil exploration and mining. As these detectors find more uses, increasing demands are placed on the electronic components of the detectors. The front end readout electronics or data acquisition system for a CZT detector is generally expected to exhibit low-noise, high linearity, wide dynamic range, and good drive capability. In addition to these requirements, portable systems may also demand data acquisition systems that are low-power, low-cost, with a high channel count.
Primarily, front end readout electronics capture two pieces of information from the radiation detector: the energy level of the radiation and the timing of the detection. While the energy level indicates the energy spectrum of the radiation, timing information is used to determine the depth of interaction so as to provide the full 3D position sensitivity needed for image reconstruction. There have been several application-specific integrated circuits (ASICs) developed to function as the front end readout electronics for radiation detectors. However, in order to achieve a high count rate of these new applications, these ASICs typically have high power consumption, and are typically subject to baseline wander caused by a variable count rate. Baseline wander is exacerbated by an increased leakage current due to the higher count rate, thus causing stability issues for high speed data acquisition. These ASICs also tend to be large in physical size, which can limit applications in which they are desired to be used.
It would be desirable to have a data acquisition system for radiation detectors that can operate at low power, with little noise, offer complete energy level and time discrimination capabilities, provide digital outputs, reduce package size, and reduce baseline wander caused by a variable count rate application.